The present invention relates to application of Ellipsometry to in-situ real-time monitoring and/or process control; and more precisely to the monitoring and/or controlling of the fabrication of multiple high/low refractive index layer narrow passband optical filters using oblique angle-of-incidence spectroscopic ellipsometric DELTA determination over a wavelength range, around said narrow passband center wavelength, in which wavelength range reflectivity is essentially 100%. Said method is optionally enhanced by combination with essentially normal angle-of-incidence non-ellipsometric transmission extrema turning point data.
The use of Spectroscopic Ellipsometry (SE) to non-invasively characterize properties, (such as thickness, composition, morphology and optical constants), of thin films ex-situ is well known. And, while less common, application to real-time in-situ fabrication monitoring and control is also known, particularly in the semiconductor area. Further, it is known that said techniques are directly applicable to investigating sample systems comprised of multiple thin film layers.
Ellipsometry basically monitors a change in Polarization State of a beam of electromagnetism, which polarization state change occurs as a result of interaction with a sample system. Based upon said change in polarization state, sample system characterizing ellipsometric PSI ("psgr") and ellipsometric DELTA (xcex94), which are defined by:
Tan(4)eixcex94=xcfx81=xcex93p/xcex93s
where rp and rs can be complex Fresnel reflectivities for xe2x80x9cpxe2x80x9d and xe2x80x9csxe2x80x9d polarized components, can be determined. It is noted that Rho (xcfx81) is a complex number defined as the ratio of the reflectivity of xe2x80x9cp-polarizedxe2x80x9d to reflectivity of the xe2x80x9cs-polarizedxe2x80x9d components of a beam of polarized electromagnetic radiation. In polar form, Tan("psgr") corresponds to the magnitude of the reflectivity ratio and (xcex94) corresponds to the phase angle introduced between xe2x80x9cpxe2x80x9d and xe2x80x9csxe2x80x9d polarized components by interaction with the sample system. Further, it should be understood that said xe2x80x9cpxe2x80x9d component is defined as being in the plane of an incident and reflected beam of electromagnetic radiation, which plane also contains a normal to the surface of a the reflective surface of the sample system. And the xe2x80x9csxe2x80x9d component is defined as being perpendicular to the direction of the xe2x80x9cpxe2x80x9d component and also parallel to said reflective surface of the sample system.
It should be appreciated that ellipsometry determines a ratio of xe2x80x9cpxe2x80x9d and xe2x80x9csxe2x80x9d polarization component intensity values rather than an absolute intensity value, and that ellipsometry provides phase shift information, (ie. between said xe2x80x9cpxe2x80x9d and xe2x80x9csxe2x80x9d components), which is not available from electromagnetic beam intensity reflection or transmission data, wherein change in xe2x80x9cpxe2x80x9d and xe2x80x9csxe2x80x9d polarization states are not monitored. It is further to be appreciated that said phase shift information is generally very sensitive to properties, (and changes therein), associated with ultra-thin films.
It must also be appreciated that many types of Ellipsometer systems exist, many of which sequentially comprise a source of electromagnetic radiation, a polarizer means for setting a polarization state, a means for supporting a sample system, an analyzer means for selecting a polarization state, and a detector means for receiving electromagnetic radiation and producing a signal which is proportional to its intensity. Typically at least one element in the ellipsometer system is caused to rotate during data acquisition, and said rotating element can be the polarizer means or analyzer means. A problem in applying rotating polarizer or rotating analyzer ellipsometer systems, however, is that ellipsometric DELTA""s of 0.0 or 180 degrees are impossible to measure therewith without use of means such as the J.A. Woollam Co. Autoretarder, (see U.S. Pat. Nos. 5,757,494 and 5,956,145). In that light it is disclosed that a relevant benefit exists where the polarizer means and analyzer means are both held essentially stationary during data acquisition, and instead an additional element, (ie. a compensator), is present and rotated during said data acquisition. The reason for this is that an important benefit is associated with rotating compensator ellipsometers in that ellipsometric DELTA""s are measurable thereby over the entire range of 0.0-360 degrees. In addition, although not as important to the present invention, rotating compensator ellipsometor systems can still measure ellipsometric PSI""s the entire range of 0.0 to 90 degrees. The J.A. Woollam Co. xe2x80x9cM2000xe2x80x9d (Reg. Trademark), Rotating Compensator Ellipsometer System is described in U.S. Pat. No. 5,872,630.
Continuing, of growing importance is the fabrication and application of Narrow Bandpass Optical Filters. Said Narrow Bandpass Optical Filters, which provide very sharp cutoff characteristics, (eg. said passbands providing a bandwidth of a nanometer or so with combined high and low transition to cutoff being less than a nm), are typically comprised of up to a hundred or more layers of alternating quarter-wavelength thick high, and quarter wavelength thick low, refractive index materials, said sequence being beneficially interspersed with half-wavelength thick cavities and/or coupling layers. Present manufacturing techniques typically control deposition of the layers of alternating quarter-wavelength high, and quarter wavelength low, optically thick refractive index materials utilizing transmission data, wherein a cyclic pattern of Transmission vs. Layer Number xe2x80x9cextremaxe2x80x9d turning points are used to determine when to change from depositing low to high, and vice-verse, refractive index material. A problem with this approach is that in some ranges said Transmission data can be relatively insensitive to change in thickness of deposited material.
In view of known prior art, there remains need for improved methodology for monitoring and/or controlling fabrication of multiple layer Narrow Bandpass Optical Filters.
The present invention is based in the discovery that, over a limited range of wavelengths surrounding the central passband wavelength of a Narrow Bandpass Optical Filter, ellipsometric DELTA""s which are determined using data obtained by oblique angle spectroscopic ellipsometric investigation, are very well behaved and that high and low refractive index materials demonstrate easily differentiated ellipsometric DELTA vs. Wavelength plots. Further, ellipsometric DELTA""s are related to optical thickness of the surface layer of a Narrow Bandpass Optical Filter during fabrication, with minimal influence on said DELTA""s being effected by previously deposited layers.
It is noted that presently Transmission Intensity Extrema Turning Point vs. Layer Number data is typically utilized during fabrication of Narrow Bandpass Optical Filters, to provide generally good insight as to when to change from deposition of high to low, and low to high refractive index materials, when quarter-wavelength optically thick layers are being formed. However, where other than quarter-wavelength optical thickness layers are being formed said transmission extrema turning point data is not sufficient to provide reliable data upon which can be based said decision. It is in that light that the present invention teaches supplementing said conventional Transmission Intensity Extrema Turning Point vs. Layer Number data with ellipsometric DELTA vs. wavelength data to improve fabrication precision.
The present invention methodology can however, be applied alone, (ie. not in combination with conventional Transmission Intensity Extrema Turning Point vs. Layer Number data). The present invention then teaches use of ellipsometric DELTA vs. wavelength data alone to monitor and/or control deposition of quarter-wavelength, and non-quarter wavelength, optical thickness layers. This is enabled as said ellipsometric DELTA data provides insight to the optical thickess of a layer of material, be it a high or low refractive index material, being deposited, said layer of material being immediately at the surface regardless of the stage of fabrication. That is, the ellipsometric DELTA data minimally sensitive to previously deposited layers.
A present invention method of monitoring and/or controlling fabrication of Narrow Bandpass Optical Filters which transmit a central passband of wavelengths while rejecting wavelengths outside thereof, then comprises the steps of;
a. providing a system for fabricating Narrow Bandpass Optical Filters which comprises means for depositing alternating layers of High and Low refractive Index materials onto a substrate; and
b. providing a spectroscopic ellipsometer system;
c. applying said spectroscopic ellipsometer system to said system for fabricating Narrow Bandpass optical Filters such that a substrate can be monitored in a reflective mode, by a beam of polarized spectroscopic electromagnetic radiation provided by said spectroscopic ellipsometer system, said spectroscopic ellipsometer system being oriented so as to direct a beam of polarized spectroscopic electromagnetic radiation upon said substrate surface at an oblique angle;
d. placing a substrate into said system for fabricating Narrow Bandpass Optical Filters;
e. while depositing a plurality of alternating High and Low Refractive Index layers onto said substrate, in real-time determining the Ellipsometric DELTA;
f. utilizing the resulting Ellipsometric DELTA data to monitor and/or control the fabrication process.
Said method can further comprise, in combination with step b, the steps of:
bxe2x80x2. providing an essentially monochromatic central passband wavelength source of electromagnetic radiation, and a detector thereof, and
cxe2x80x2. orienting said essentially central passband monochromatic wavelength source of electromagnetic radiation and detector thereof such that a beam of electromagnetic radiation is provided by said essentially monochromatic central passband wavelength source of electromagnetic radiation substantially along a normal to the surface of the substrate, transmits through said substrate and enters said detector.
When said steps bxe2x80x2. and cxe2x80x2. are added the method then further comprises obtaining real-time electromagnetic radiation Transmission data, and utilizing the results thereof in combination with the spectroscopic ellipsometric data in practicing step f. This can involve, for instance, using the transmission data to determine a sequence of extrema turning points in a plot of transmission intensity vs. layer number, and using said extrema turning points to determine when to change from depositing high to low and low to high refractive index materials when depositing quarter-wavelength optical thickness layers. The Ellipsometric DELTA data, which typically comprises a sequence of DELTA values vs. wavelength, is used to determine when to change from depositing high to low and low to high refractive index materials when depositing other than quarter-wavelength optical thickness layers, and to supplement the transmission data where said data indicates low sensitivity to quarter-wave optical thickness layer fabrication results.
A preferred embodiment of the recited method of monitoring and/or controlling fabrication of Narrow Bandpass Optical Filters further comprises a step of determining a wavelength range, around the central bandpass wavelengths, in which reflectivity of the Narrow Bandpass Optical Filter being fabricated is expected to be essentially 100%, and during the step e. determination of Ellipsometric DELTA, limiting determination of the ellipsometric DELTA using only that range of wavelengths. Said preferred embodiment can also include excluding the central passband wavelengths during said ellipsometric DELTA determination.
A more detailed recitation of the present invention methodology for monitoring and/or controlling fabrication of Narrow Bandpass Optical Filters which transmit a central passband of wavelengths while rejecting wavelengths outside thereof, to comprises the steps of;
a. providing a system for fabricating Narrow Bandpass Optical Filters which comprises means for depositing alternating layers of High and Low refractive Index materials onto a substrate; and
b. providing a spectroscopic ellipsometer system and an essentially monochromatic transmission system;
c. applying said spectroscopic ellipsometer system to said system for fabricating Narrow Bandpass Optical Filters such that a substrate can be monitored in a reflective mode, by a beam of polarized spectroscopic electromagnetic radiation provided by said spectroscopic ellipsometer system, said spectroscopic ellipsometer system being oriented so as to direct a beam of polarized spectroscopic electromagnetic radiation onto said substrate surface at an oblique angle;
cxe2x80x2. applying said essentially monochromatic transmission system to said system for fabricating Narrow Bandpass Optical Filters such that a substrate can be monitored in a transmissive mode by a beam of electromagnetic radiation which is provided by said essentially monochromatic central passband wavelength source of electromagnetic radiation, said essentially monochromatic transmission monitoring system being oriented to direct a beam of essentially monochromatic radiation substantially along a normal to the surface of the substrate;
d. placing a substrate into said system for fabricating Narrow Bandpass Optical Filters;
e. determining a wavelength range around a central band pass wavelength of the Narrow Bandpass Optical Filter in which reflectivity thereof to a beam of electromagnetic radiation impinged at an oblique angle to a surface thereof is expected to to be essentially 100%;
f. while depositing a plurality of alternating High and Low Refractive Index layers onto said substrate, determining both transmission data in the central passband wavelengths, and Ellipsometric DELTA in the range of wavelengths identified in step e., said determined transmission data comprising a sequence of extrema turning points in a plot of transmission intensity vs. layer number, and said determined ellipsometric DELTA data comprising a sequence of DELTA values vs. wavelength;
g. utilizing the resulting Transmission and Ellipsometric DELTA data to monitor and/or control the fabrication process, said extrema turning points being used to determine when to change from depositing high to low and low to high refractive index materials when depositing quarter-wavelength optical thickness layers, and said ellipsometric DELTA data being used to determine when to change from depositing high to low and low to high refractive index materials when depositing other than quarter-wavelength optical thickness layers;
Again, said method of monitoring and controlling fabrication of Narrow Bandpass Optical Filters can include excluding the central passband wavelengths from the range of wavelengths over which the ellipsometric DELTA is determined as identified in step e., and utilized in step f.
It is disclosed that, while not limiting, a preferred spectroscopic ellipsometer system for application in practicing the present invention is a rotating compensator sample system investigation system comprising a source of a polychromatic beam of electromagnetic radiation, a polarizer, a stage for supporting a sample system, an analyzer, a dispersive optics and at least one detector system which contains a multiplicity of detector elements. In addition, said spectroscopic rotating compensator sample system investigation system further comprises at least one compensator(s) positioned at a location selected from the group consisting of: (before said stage for supporting a sample system, and after said stage for supporting a sample system, and both before and after said stage for supporting a sample system). When said spectroscopic rotating compensator sample system investigation system is used to investigate a sample system present on said stage for supporting a sample system, said analyzer and polarizer are maintained essentially fixed in position and at least one of said at least one compensator(s) is caused to continuously rotate while a polychromatic beam of electromagnetic radiation produced by said source of a polychromatic beam of electromagnetic radiation is caused to pass through said polarizer and said compensator(s), said polychromatic beam of electromagnetic radiation being also caused to interact with said sample system, pass through said analyzer and interact with said dispersive optics such that a multiplicity of essentially single wavelengths are caused to simultaneously enter a corresponding multiplicity of detector elements in said at least one detector system. The described rotating compensator ellipsometer system is sold by the J.A. Woollam CO. under the Trademark M2000, and is protected by U.S. Pat. No. 5,872,630.
The present invention will be better understood by reference to the Detailed Description Section of this Specification, in combination with the Drawings.
It is therefore a primary purpose and/or objective of the present invention to teach a method which is useful in the fabrication of multiple layer high/low refractive index, narrow bandpass optical filter structures, which method utilizes spectroscopic ellipsometry.
In conjunction with the first purpose and/or objective, it is further a purpose and/or objective of the present invention to teach:
use of ellipsometric DELTA vs. Wavelength data determined at an oblique angle of incidence over a wavelength range around a central passband wavelength of a narrow bandpass optical filter, in which wavelength range the reflectivity is essentially 100%,
in monitoring and/or controlling the fabrication of multiple layer high/low refractive index, narrow bandpass optical filter structures.
It is a further purpose and/or objective of the present invention to teach combined use of:
ellipsometric DELTA vs. Wavelength data determined at an oblique angle of incidence over a wavelength range around a central passband wavelength of a narrow bandpass optical filter, in which wavelength range the reflectivity is essentially 100%, from which wavelength range the central passband wavelengths are optionally excluded, and
transmission extrema turning point data achieved at said central passband wavelength;
in monitoring and/or controlling the fabrication of multiple layer, high/low refractive index, narrow bandpass optical filter structures.
Other objectives and/or purposes will become apparent from a reading of the Specification and Claims.